The present invention relates to the art of diagnostic medical imaging. It finds particular application in conjunction with magnetic resonance imaging (MRI), and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
In MRI, a substantially uniform temporally constant main magnetic field, B.sub.0, is generated within an examination region. The main magnetic field polarizes the nuclear spin system of a subject being imaged within the examination region. Magnetic resonance is excited in dipoles which align with the magnetic field by transmitting radio frequency (RF) excitation signals into the examination region. Specifically, RF pulses transmitted via an RF coil assembly tip the dipoles out of alignment with the main magnetic field and cause a macroscopic magnetic moment vector to precess around an axis parallel to the main magnetic field. The precessing magnetic moment, in turn, generates a corresponding RF magnetic resonance signal as it relaxes and returns to its former state of alignment with the main magnetic field. The RF magnetic resonance signal is received by the RF coil assembly, and from received signals, an image representation is reconstructed for display on a human viewable display.
Different tissues of the body have different pairs of relaxation properties that are characterized by a pair of time constants: T1 which is the spin-lattice relaxation time, and T2 which is the spin-spin relaxation time. Therefore, different images and visualization of different anatomical structures are obtained depending upon the time constant most heavily relied upon. In this regard, a T1 weighted image is one in which the intensity contrast between any two tissues in an image is due mainly to the T1 relaxation properties of the tissue, and a T2 weighted image is one in which the intensity contrast between any two tissues in an image is due mainly to the T2 relaxation properties of the tissue.
In any event, the appropriate frequency for exciting resonance in selected dipoles is governed by the Larmor equation. That is to say, the precession frequency of a dipole in a magnetic field, and hence the appropriate frequency for exciting resonance in that dipole, is a product of the gyromagnetic ratio .gamma. of the dipole and the strength of the magnetic field. In a 1.5 T magnetic field, hydrogen (.sup.1 H) dipoles have a resonance frequency of approximately 64 MHZ. Generally in MRI, the hydrogen species is excited because of its abundance and because it yields a strong MR signal. As a result, typical MRI apparatus are equipped with built-in whole-body RF coils tuned to the resonant frequency for hydrogen.
For certain applications it is desirable to obtain a T.sub.1 weighted image. Moreover, at times, having a spin echo (i.e., an echo derived from application of an RF pulse) image appearance is also advantageous. There are however obstacles to overcome, such as timeliness and specific absorption rate (SAR). With regard to the SAR, SAR=Joules of RF/Second/kg of body weight=Watts/kg. When the SAR is high, it leads to unwanted heating and potential burning of body tissue. That is to say, introduction of high levels of high energy RF pulses into the patient being imaged has the potential of burning the patient.
While prior MRI techniques have been adequate for their intended purposes, certain drawbacks make them less than ideal for the task at hand. For example, the typical spin echo (SE) technique has a relatively low SAR compared to other MRI sequences. However, the main disadvantage of the typical SE technique is that the acquisition time is lengthy relative to other techniques which collect multiple echos per TR (i.e., the time to repeat, or in other words, the time between excitations). While the conventional fast spin echo (FSE) technique results in improved timeliness over the SE technique, its introduction of high energy RF refocussing pulses leads to increased SAR issues. The SAR issues can be reduced through the use of a conventional gradient and spin echo (GSE) technique. However, the disadvantage of the typical GSE technique is that the TE (i.e., the time to echo, or in other words, the time from excitation to spin echo) is longer than desirable for a T1 weighted image due to the fact that one or more field echos or gradient echos (i.e., echos generated as a result of a magnetic gradient switching polarities) are acquired prior to the spin echo. Moreover, the typical GSE technique, through its particular use of field echos, reduces the true spin echo nature of the image and introduces artifacts typically associated with field echo imaging, such as susceptibility.
The present invention contemplates a new and improved MRI technique which overcomes the above-referenced problems and others.